1. Field of the Invention
The present invention relates to a marine propulsion device.
2. Description of the Related Art
In marine propulsion devices such as the outboard motors, a drive shaft and a propeller are coupled through a rearward travel gear when a shift lever is shifted to a rearward travel position during high speed navigation. However, when the propeller is coupled to the rearward travel gear, the propeller can be rotated by a water stream passing through the propeller in a forward travel direction. Accordingly, the engine is reversely rotated. Marine propulsion devices are normally configured to discharge exhaust gas from the engine into the water through an exhaust path. Therefore, reverse rotation of the engine causes a phenomenon of inhaling water into the engine through the exhaust path. Such water intrusion into the engine may damage or break the engine.
Japan Laid-open Patent Application Publication No. JP-A-2003-120397 describes a device configured to count an angle signal of a crankshaft and detect reverse rotation of the crankshaft in accordance with the counted values. Further, the device is configured to stop ignition and fuel supply when detecting reverse rotation of the crankshaft. Thus, water intrusion into the engine is prevented by stopping the engine.
Further, Japan Laid-open Patent Application Publication No. JP-A-2008-274970 describes a device configured to detect reverse rotation of a crankshaft by detecting a crankshaft rotational angle and a cam shaft rotational angle. The device is configured to forcibly shift a forward/rearward travel switching gear to a neutral position when detecting reverse rotation of the crankshaft. Accordingly, water intrusion into the engine is prevented.
According to the device of Japan Laid-open Patent Application Publication No. JP-A-2003-120397, the propeller is rotated by the water stream even when ignition is stopped. Accordingly, reverse rotation of the crankshaft is continued. Therefore, prevention of water intrusion into the engine is relatively ineffective. According to the device of Japan Laid-open Patent Application Publication No. JP-A-2008-274970, on the other hand, the crankshaft may be continuously reversely rotated until the gear is completely shifted to the neutral position. In addition, reverse rotation of the crankshaft may be continued due to an inertial force for a while even after the gear is completely shifted to the neutral position. Therefore, prevention of water intrusion into the engine is limited. In view of the above, both devices described in the above-identified publications can prevent a large amount of water intrusion into the engine but cannot completely prevent water intrusion into the engine. In other words, both devices cannot prevent a small amount of water intrusion into the engine. When a small amount of water intrudes into the engine, the engine may be damaged or broken due to its corrosion after the elapse of a long period of time, even if the engine is not be immediately damaged or broken.